Kripke model in the context of André Joyal

An accessibility relation is a relation which plays a key role in assigning truth values to sentences in the relational semantics for modal logic. In relational semantics, a modal formula's truth value at a possible world ${\displaystyle w}$ can depend on what is true at another possible world ${\displaystyle v}$, but only if the accessibility relation ${\displaystyle R}$ relates ${\displaystyle w}$ to ${\displaystyle v}$. For instance, if ${\displaystyle P}$ holds at some world ${\displaystyle v}$ such that ${\displaystyle wRv}$, the formula ${\displaystyle \Diamond P}$ will be true at ${\displaystyle w}$. The fact ${\displaystyle wRv}$ is crucial. If ${\displaystyle R}$ did not relate ${\displaystyle w}$ to ${\displaystyle v}$, then ${\displaystyle \Diamond P}$ would be false at ${\displaystyle w}$ unless ${\displaystyle P}$ also held at some other world ${\displaystyle u}$ such that ${\displaystyle wRu}$.

Accessibility relations are motivated conceptually by the fact that natural language modal statements depend on some, but not all, alternative scenarios. For instance, the sentence "It might be raining" is not generally judged true simply because one can imagine a scenario where it is raining. Rather, its truth depends on whether such a scenario is ruled out by available information. This fact can be formalized in modal logic by choosing an accessibility relation such that ${\displaystyle wRv}$ if ${\displaystyle v}$ is compatible with the information that is available to the speaker in ${\displaystyle w}$.